Embodiments of the present invention relate to centrifugal compressor units and, more specifically, built-in centrifugal compressor units, in which the compressor and motor drive means of the compressor are assembled in a shared housing sealed against the gas handled by the compressor.
A conventional built-in compressor unit includes motor means, generally comprising an electric drive motor and a centrifugal compressor with one or more compression stages.
Each compression stage includes an impeller mounted on a driven shaft coupled to the rotor of a drive motor.
In certain applications, and in particular for low-pressure applications, the use of variable-pitch impellers has been proposed to modify the work done by the compression stage as a function of the gas flow rate. This makes it possible to keep the work of the compressor constant for a wider range of gas flow rates. Patent application FR 1061391 thus proposes placing the variable-pitch impellers both upstream of a compressor impeller and in a diffuser of a compressor stage.
Mechanical devices can be used to modify the orientation of the blades, for example by fitting a group of blades with a ring gear driven by a worm gear device, or by fitting each blade with direct drive means dedicated to that blade.
A motor is then used to actuate the mechanical device for controlling orientation of the blades.
The movable blades thus inserted in the gas flow are subject to considerable deflection stresses in relation to the axis of rotation thereof, and significant torque is required to orient each blade. The blades and the drive system thereof need to be dimensioned accordingly. Adding the movable-blade system therefore represents a cost that should ideally be reduced, said cost being even greater if the compressor has several stages.